Rendering Blender animations on a server: more RAM or faster CPU or both?
Note: I assumed that you are just running a single blender render instance on the system at the same time.

Depends on the complexity of you scenes. If these are rather simple scenes than not that much of RAM is needed. Adding more RAM will then not increase performance. More RAM will only increase performance when the machine is going to swap on disk.

If the complexity of the scenes are known, then the Amount of RAM can be received from blender easy (see top bar). Add some percentage slack and you will be fine. 

If you still want to render more quick, then just add more CPU's. Blender is optimized to scale very well by just adding more CPU. It does not have noticeable overhead on memory.

Another way to increase performance is to optimize your scenes. There are a lot of topics and tutorials on area's where you can win speed.

So to answer short: Find RAM needed for the complexity of your scenes. Then just add more CPU's.